JP2016171419A - Cpt resonance generation method, cpt resonance detection method, cpt resonance generator, atomic oscillator, magnetic sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a CPT (Coherent Population Trapping) resonance generation method of small light shift.SOLUTION: In a CPT resonance generation method for generating laser light having at least two wavelengths by current injection into a laser light-emitting element, and irradiating an alkaline metal with the laser light, the value of the DC component of a current applied to the laser light-emitting element is larger than the oscillation threshold of the laser light-emitting element in a first period, and smaller than the value of the DC component of a current in the first period, in a second period following the first period, and Ramsey resonance is generated by repeating the first and second periods a plurality of times.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a CPT resonance generation method, a CPT resonance detection method, a CPT resonance generation apparatus, an atomic oscillator, and a magnetic sensor.

  There is an atomic clock as a clock for measuring extremely accurate time, and a technique for downsizing the atomic clock has been studied. An atomic clock is an atomic oscillator based on the amount of transition energy of electrons constituting atoms such as alkali metals. Since the transition energy of electrons in an alkali metal atom is very precise in the absence of disturbance, an atomic oscillator can obtain frequency stability several orders of magnitude higher than that of a crystal oscillator.

  Conventional atomic oscillators require a microwave resonator and are therefore large and consume a large amount of power. However, the use of atomic resonance called Coherent Population Trapping (hereinafter referred to as CPT) eliminates the need for a resonator and makes it possible to fabricate a very small atomic oscillator. In 2007, a prototype CPT type atomic oscillator was manufactured, and in 2011, consumer products were sold by Symmetricom.

  In the CPT type atomic oscillator, as shown in FIG. 1, a light source 910 such as a laser light emitting element, an alkali metal cell 940 encapsulating an alkali metal, and a photodetector 950 that receives laser light transmitted through the alkali metal cell 940. And have. The laser light from the light source 910 is modulated, and two transitions of electrons in the alkali metal atom are simultaneously performed and excited by sideband wavelengths appearing on both sides of the carrier wave having a specific wavelength.

  The transition energy in this transition is invariant, and when the sideband wavelength of the laser light coincides with the wavelength corresponding to the transition energy, a transparency phenomenon occurs in which the light absorption rate in the alkali metal is lowered. Therefore, the wavelength of the carrier wave is adjusted so that the light absorption rate by the alkali metal is lowered, and the signal detected by the photodetector 950 is fed back to the modulator 960, and the light source 910 such as a laser element is fed by the modulator 960. The modulation frequency of the laser beam from is adjusted. Note that laser light is emitted from the light source 910, is irradiated onto the alkali metal cell 940 through the collimating lens 920 and the quarter-wave plate 930, and is incident on the photodetector 950.

  By the way, although it is a CPT-type atomic oscillator that achieves smaller power consumption than the conventional type, the frequency stability has not yet reached the characteristics of the conventional type, and further improvement is desired. As a promising method for improving the frequency stability, a method of pulsing laser light has been studied.

  In a CPT type atomic oscillator, there are roughly two methods of pulsing laser light. The first method for pulsing the laser light is a method using an external device. In the first method, the laser light-emitting element is always made to emit light, and the wavelength of the laser light-emitting element is stabilized by matching the absorption line of the atom, and an AOM (Acousto-Optic Modulator), a liquid crystal polarizer, etc. Pulsed through an external device. In the first method, it is easy to stabilize the laser wavelength, but there is a problem of increase in volume, cost, and power consumption due to the use of an external device.

  A second method for pulsing the laser light is a method using direct modulation. In the second method, a special device other than the laser light emitting element is not required, and the entire device can be reduced in size, cost, and power consumption compared with the case of using an external device. As an example of the second method, a technique has been proposed in which a microwave is superimposed on a direct current input to a laser light emitting element to perform modulation (see, for example, Patent Document 1).

  However, the technique described in Patent Document 1 has a period in which a microwave is superimposed on the direct current Idc and applied to the laser light emitting element, and a subsequent period in which only the direct current Idc is applied to the laser light emitting element. Yes. In other words, the same amount of direct current component (DC current Idc) as that during the period in which the microwave current is superimposed is applied to the laser light emitting element even during the period in which the microwave current is not superimposed, and the laser light is blocked. Absent. Therefore, there has been a problem that a write shift occurs.

  Light shift is a phenomenon in which the energy level of alkali metal atoms changes due to the interaction between alkali metal atoms and the laser light photoelectric field, and the resonance frequency shifts. In contrast, it is linear. The write shift is a factor that lowers long-term stability in a CPT type atomic oscillator.

  The present invention has been made in view of the above, and an object of the present invention is to provide a method for generating CPT resonance with a small write shift.

  The present CPT resonance generation method is a CPT resonance generation method in which laser light having at least two wavelengths is generated by current injection into a laser light emitting element, and an alkali metal is irradiated with the laser light, which is applied to the laser light emitting element. The value of the direct current component of the current to be generated is greater than the oscillation threshold of the laser light emitting element in the first period, and in the second period following the first period, the direct current component of the current in the first period It is a requirement that the Ramsey resonance is generated by repeating the first period and the second period a plurality of times.

  According to the disclosed technology, it is possible to provide a CPT resonance generation method with a small light shift.

It is explanatory drawing of an atomic oscillator. It is a block diagram which illustrates the basic composition of the CPT resonance generator concerning a 1st embodiment. It is explanatory drawing of the observation method of CPT resonance based on the pulse excitation method. It is explanatory drawing of the relationship between the input current and output wavelength of VCSEL. It is a block diagram which shows the structure of the apparatus used for experiment. It is explanatory drawing of the atomic energy level relevant to a CPT system. It is explanatory drawing of the output wavelength at the time of VCSEL modulation. FIG. 5 is a correlation diagram between a modulation frequency and a transmitted light amount. It is a figure which shows the contrast characteristic obtained from experiment. It is a figure which shows the resonance line | wire width characteristic obtained from experiment. It is a figure which shows the performance index obtained from experiment. It is a figure which illustrates the structure of the atomic oscillator which concerns on 2nd Embodiment.

  Hereinafter, embodiments for carrying out the invention will be described with reference to the drawings. In the drawings, the same components are denoted by the same reference numerals, and redundant description may be omitted.

<First Embodiment>
In the first embodiment, a CPT resonance generator, a CPT resonance generation method and a CPT resonance detection method using the same will be described.

  FIG. 2 is a block diagram illustrating a basic configuration of the CPT resonance generator according to the first embodiment. Referring to FIG. 2, the CPT resonance generator 1 includes a laser light emitting element 3, an alkali metal cell 7, and a power supply device 9.

  The CPT resonance generator 1 uses a direct modulation method in which a pulse current is input to the laser light emitting element 3 from the power supply device 9 to directly pulse the output of the laser light emitting element 3. Since the CPT resonance generator 1 does not require a special device other than the laser light emitting element 3, the entire device can be reduced in size, cost, and consumption compared to the case of using an external device (AOM, liquid crystal polarizer, etc.). Electric power can be reduced. Hereinafter, the operation of the CPT resonance generator 1 will be described in detail.

  The laser light emitting element 3 is a light source that irradiates the alkali metal cell 7 with laser light. As the laser light emitting element 3, for example, a surface emitting laser (VCSEL: Vertical Cavity Surface Emitting LASER) can be used.

  The alkali metal cell 7 is a cell in which an alkali metal gas is sealed. As the alkali metal, for example, rubidium (Rb), cesium (Cs), sodium (Na), potassium (K) or the like can be used. A buffer gas may be enclosed in the alkali metal cell 7 together with a gas of alkali metal atoms.

  The power supply device 9 is a device that applies a current to the laser light emitting element 3. The power supply device 9 has a function of generating a pulse waveform, a function of adding a high-frequency (eg, 4.6 GHz) modulation signal to the pulse waveform, and the like. The power supply device 9 can cause the laser light emitting element 3 to generate laser light having at least two wavelengths. This will be described later with reference to FIGS.

  Pulse excitation can be performed by causing the power source device 9 to emit the laser light emitting element 3 and irradiating the alkali metal cell 7 with laser light. Here, the pulse excitation is a method of causing Ramsey resonance by irradiating the atom a plurality of times with a time interval. By applying this method, it is possible to narrow the line width of CPT resonance and improve the S / N ratio (contrast), and to improve short-term stability.

  Note that Ramsey resonance means that when a laser is irradiated multiple times with a time interval, the probability that the atom will transition to another energy level due to the interaction between the laser and the atom is sensitive to changes in the laser frequency. It is a phenomenon that becomes. By using this phenomenon, the transition frequency can be measured with high accuracy.

  FIG. 3 is an explanatory diagram of an observation method of CPT resonance based on the pulse excitation method. As shown in FIG. 3, in the case of performing pulse excitation using pulsed light that periodically repeats ON / OFF, after sufficiently exciting the CPT resonance, the laser light is cut off to allow the free development time T to elapse. The Ramsey resonance becomes observable by performing observations at.

Therefore, observation is performed after τ ₀ from the rising edge of the pulse, and then the atoms are sufficiently excited until the end of the pulse. The conditions required at this time are as follows: first, the atoms are sufficiently excited at the end of the pulse, second, the observation timing τ ₀ is sufficiently early, and third, the laser wavelengths at the time of excitation and observation are equal. . However, in the pulse excitation method shown in FIG. 3, it is difficult to sufficiently achieve the second and third conditions.

  Therefore, in the present embodiment, the second and third conditions are achieved by improving the pulse excitation method of FIG. Specifically, as shown in FIG. 4A, the Ramsey resonance is generated by repeating the first period τ and the second period T a plurality of times.

  The first period τ is a period for causing the laser light emitting element 3 to emit light, and has a pulse waveform such that the value of the direct current component of the current applied to the laser light emitting element 3 is larger than the oscillation threshold value of the laser light emitting element 3. Applied.

Specifically, in the first period τ, as the input current of the laser light emitting element 3, a current I that overdrives the vicinity of the normal pulse current rise of the laser light emitting element 3 is applied from the power supply device 9. Current I can be configured to include a current I ₁ of the first stage just after rising, the second stage of a current value smaller than the current I ₁ of the first stage and the current I _2. Note that the current values of the current I ₁ and the current I ₂ are both larger than the oscillation threshold value of the laser light emitting element 3.

By inputting a current larger than the normal pulse current of the laser light emitting element 3 as the first stage current I ₁ , the rise in the internal temperature of the laser light emitting element 3 is accelerated and the rise time of the output wavelength is greatly shortened. be able to. Thereafter, the current is changed to a second-stage current I ₂ smaller than the first-stage current I _1, and control is performed so that the output wavelength of the laser light emitting element 3 matches the absorption line L in the vicinity of the pulse termination.

  In the second period T (free development state) following the first period τ, the value of the direct current component of the current applied to the laser light emitting element 3 is smaller than the value of the direct current component of the current in the first period τ. Is set. In the second period T, the value of the direct current component of the current applied to the laser light emitting element 3 is preferably smaller than the oscillation threshold value of the laser light emitting element 3, and the value of the direct current component may be zero.

  Thus, the value of the direct current component of the current applied to the laser light emitting element 3 in the second period T is made smaller than the value of the direct current component of the current applied to the laser light emitting element 3 in the first period τ. As a result, the amount of laser light in the second period T decreases. Therefore, the write shift can be reduced.

  In particular, the value of the direct current component of the current applied to the laser light emitting element 3 in the second period T is set to a value smaller than the oscillation threshold value of the laser light emitting element 3 (including the case where it is 0). During the period T, the laser light emitting element 3 does not emit light. For this reason, it is possible to completely block the laser beam irradiated to the alkali metal atoms, and the light shift can be greatly reduced.

As shown in FIG. 4B, in the first period τ, the output wavelength of the laser light emitting element 3 increases from the value larger than the absorption line L and the absorption line L from the value larger than the absorption line L. And the process of decreasing. In other words, the output wavelength of the laser light emitting element 3 once passes through the absorption line L after τ ₀ from the rising edge of the pulse, and then follows a locus that matches the absorption line L again.

In the process of increasing the output wavelength of the laser light emitting element 3 in FIG. 4B, when the output wavelength of the laser light emitting element 3 matches the absorption line L, CPT resonance can be detected. Thus, by detecting CPT resonance at the timing of τ ₀ that first passes through the absorption line L, the second and third conditions necessary for observation of pulse excitation can be achieved. Here, the absorption line L is the absorption wavelength of the alkali metal sealed in the alkali metal cell 7.

In the second period T, if a current smaller than the oscillation threshold of the laser light emitting element 3 is allowed to flow without completely blocking the direct current component of the current injected into the laser light emitting element 3, the laser light emitting element 3 emits light again. Thus, the rising speed of the output wavelength when the CPT resonance is generated can be improved. As a result, a shorter observation timing τ ₀ can be realized.

  As described above, in the CPT resonance generation method using the CPT resonance generation apparatus 1 according to the first embodiment, the light shift can be reduced as compared with the conventional method. Shown by experiment.

[Experimental device]
An experimental apparatus shown in FIG. 5 having the basic configuration of the CPT resonance generator according to the first embodiment was manufactured, and experiments related to CPT resonance generation and CPT resonance detection were performed.

  In FIG. 5, a VCSEL 311 is used as a laser light emitting element for excitation. The temperature of the VCSEL 311 is kept constant. A power supply device configured by the current driver 312, the PLL 314, and the bias circuit 315 generates a current obtained by adding a 4.6 GHz modulation signal to the current waveform in FIG. 4A and applies it to the VCSEL 311. A control signal is input to the PLL 314 from the control unit 316.

  The alkali metal cell 317 has a cylindrical shape with a diameter of 20 mm and an optical path length of 10 mm, and is filled with 4 kPa of Cs which is an alkali atom and Ne which is a buffer gas. The temperature of the alkali metal cell 317 is kept at 39.00 ° C. where the S / N ratio of CPT resonance is the highest. A static magnetic field B is applied to the alkali metal cell 317 in order to block an external magnetic field due to geomagnetism or the like.

  Laser light from the VCSEL 311 is irradiated to the alkali metal cell 317, and transmitted light is detected by a photodiode 318 which is a photodetector. The signal of the photodiode 318 is amplified after being lock-in detected by the lock-in amplifier 319 and fed back to the current driver 312 in order to stabilize the wavelength of the laser light emitted from the VCSEL 311. A function generator 313 is connected to the lock-in amplifier 319 and the current driver 312.

In order to detect CPT resonance, the signal of the photodiode 318 is connected to a measuring instrument 322 such as an oscilloscope via a sample hold circuit 321. A pulse generator 323 is connected to the sample hold circuit 321. By using the sample hold circuit 321, CPT resonance detection at τ ₀ in FIG. 4 can be performed.

  In this experimental apparatus, as shown in FIG. 6, when Cs atoms enclosed in an alkali metal cell 317 are excited simultaneously from two ground levels A and B to an excited level C, the light absorption rate is obtained. Take advantage of the decline. Therefore, an element having a carrier wavelength close to 894.6 nm is used as the VCSEL 311. The wavelength of the carrier wave can be tuned by changing the temperature or output of the VCSEL 311.

  In this experimental apparatus, laser light having at least two wavelengths is generated. Specifically, as shown in FIG. 7, by frequency-modulating the VCSEL 311, sidebands are generated on both sides of the carrier near 894.6 nm, and the frequency difference is 9.2 GHz which is the natural frequency of the Cs atom. Is modulated at 4.6 GHz so as to match. Laser light having two different wavelengths as sidebands is incident on the alkali metal cell 317.

  As shown in FIG. 8, the amount of transmitted laser light that passes through the excited Cs gas in the alkali metal cell 317 is maximized when the sideband frequency difference matches the natural frequency difference of Cs atoms. Therefore, the modulation frequency in the VCSEL 311 is adjusted by performing feedback so that the output of the photodiode 318 maintains the maximum value. Since the natural frequency of the atom is extremely stable, the modulation frequency becomes a stable value, and this information can be extracted as an output.

As described above, in this experimental apparatus, a current obtained by adding a modulation signal of 4.6 GHz to the current waveform of FIG. 4A is injected into the VCSEL 311, and laser light having at least two wavelengths is passed through the alkali metal cell 317. Irradiate Cs atoms. Then, in the first period τ shown in FIG. 4B, the light that has passed through the alkali metal cell 317 at the timing of τ ₀ that first passes through the absorption line L is detected by the photodiode 318.

  In this experimental apparatus, Cs is used as the alkali metal, and a VCSEL having a wavelength of 894.6 nm is used to use the transition of the D1 line. However, when using the C2 D2 line, a VCSEL of 852.3 nm is used. Can be used. Rb can also be used as the alkali metal, and a VCSEL of 795.0 nm can be used when the D1 line is used, and a 780.2 nm VCSEL can be used when the D2 line is used. The modulation frequency when using Rb is 3.4 GHz for 87Rb and 1.5 GHz for 85Rb.

[Experimental result]
(Experimental result 1. Contrast)
First, using the experimental apparatus shown in FIG. 5, the contrast characteristics were compared between the CPT resonance using the pulse excitation shown in FIG. 4 (hereinafter referred to as the present excitation method) and the conventional CPT resonance by continuous excitation. . The contrast is an index generally used to express the S / N ratio of CPT resonance, and is the ratio between the resonance amplitude and the DC component. The higher the contrast, the better the short-term stability.

  FIG. 9 shows contrast characteristics obtained from experiments. In FIG. 9, the rhombus plot is the contrast when the present excitation method is used, and the broken line plot is the contrast when continuous excitation is used (the same applies to FIGS. 10 and 11). In this excitation method, the characteristics when the second period T (free development time) shown in FIG. 4 is changed are measured (the same applies to FIGS. 10 and 11).

  As shown in FIG. 9, the contrast was about 2.19% in continuous excitation, whereas the maximum contrast was 3.57% in this excitation method. In other words, this excitation method can provide higher contrast than continuous excitation and can be expected to improve short-term stability.

(Experimental result 2. Resonance line width)
Next, the resonance linewidth (Full Width Half Maximum: FWHM) characteristics of the CPT resonance using this excitation method and the conventional CPT resonance by continuous excitation were compared. Note that the narrower the resonance line width, the better the short-term stability.

  FIG. 10 shows the resonance line width characteristics obtained from the experiment. As shown in FIG. 10, the resonance line width was about 2.96 kHz in the continuous excitation, whereas the resonance line width was 405 Hz at the minimum in this excitation method. That is, in this excitation method, a resonance line width that is significantly narrower than that of continuous excitation can be obtained, and an improvement in short-term stability can be expected.

(Experimental result 3. Short-term stability figure of merit)
Next, a short-term stability figure of merit was compared for CPT resonance using this excitation method and CPT resonance by conventional continuous excitation.

  Alan standard deviation is generally used as an index for expressing the frequency stability of CPT resonance. The short-term stability of the Allan standard deviation is proportional to the value obtained by dividing the resonance line width by the contrast, and the smaller the value, the better the characteristics. Therefore, the value obtained by dividing the contrast by the resonance line width is used as the figure of merit.

  FIG. 11 shows the figure of merit obtained from the experiment (the value normalized with the figure of merit of continuous excitation as 1 is used). As shown in FIG. 11, the figure of merit of this excitation method was 6.72 at the maximum with respect to continuous excitation. That is, it was found that the present excitation method can expect a performance improvement of 6.72 times compared to the continuous excitation, and the effectiveness of this excitation method was confirmed.

<Second Embodiment>
In the second embodiment, an atomic oscillator including the CPT resonance generator according to the first embodiment as a basic configuration will be exemplified. In the second embodiment, description of the same components as those of the already described embodiments may be omitted.

  FIG. 12 is a diagram illustrating the structure of the atomic oscillator according to the second embodiment. Referring to FIG. 12, the atomic oscillator 10 includes, as main components, a laser light emitting element 30, an ND (Neutral Density) filter 40, a collimator lens 50, a quarter-wave plate 60, an alkali metal cell 70, light detection. Device 80, modulator 200, and the like.

  In the atomic oscillator 10, the light emitted from the laser light emitting element 30 is irradiated to the alkali metal cell 70 through the ND filter 40, the collimator lens 50, and the quarter wavelength plate 60 and enclosed in the alkali metal cell 70. Excited electrons in the alkali metal atom. The light transmitted through the alkali metal cell 70 is received by the photodetector 80 which is a light receiving unit, and the signal received by the photodetector 80 is fed back to the modulator 200, and the modulator 200 modulates the laser light emitting element 30. .

  Hereinafter, the structure of the atomic oscillator 10 will be described in more detail. In the present embodiment, for convenience, the photodetector 80 side of the atomic oscillator 10 is the upper side, and the package 110 side described later is the lower side. However, the atomic oscillator 10 can be used upside down or arranged at an arbitrary angle. Further, the plan view means that the object is viewed from the traveling direction of the excitation light.

  The atomic oscillator 10 has a circuit board 15, and main components are formed on the circuit board 15 in the vertical direction. Specifically, an alumina substrate 20 is provided on the circuit board 15, and a laser light emitting element 30 is provided on the alumina substrate 20. As the laser light emitting element 30, for example, a surface emitting laser (VCSEL) or the like can be used. The alumina substrate 20 is provided with a heater 25 for controlling the temperature of the laser light emitting element 30.

  An ND filter 40 is installed at a predetermined position above the laser light emitting element 30 through a heat insulating spacer 101 formed of glass or the like. A collimator lens 50 is installed on the upper surface of the ND filter 40. A quarter-wave plate 60 is installed at a predetermined position above the ND filter 40 via a spacer 102 made of silicon or the like.

  An alkali metal cell 70 is installed at a predetermined position above the quarter-wave plate 60 through a heat insulating spacer 103 made of glass or the like. The alkali metal cell 70 has a structure in which the outer edges of two glass substrates 71 facing each other are bonded together via a silicon substrate 72, and an alkali metal atom is formed in a portion surrounded by the glass substrate 71 and the silicon substrate 72. Of gas is enclosed.

  Examples of the alkali metal are as described above (Cs and the like). Note that a buffer gas such as Ne may be enclosed in the alkali metal cell 70 together with a gas of alkali metal atoms. On both sides of the alkali metal cell 70, cell heater wiring is provided on the surface of the glass substrate 71, and the alkali metal cell 70 can be set to a predetermined temperature.

  A photodetector 80 is installed at a predetermined position above the alkali metal cell 70 via a heat insulating spacer 104 formed of glass or the like. As the photodetector 80, for example, a photodiode or the like can be used.

  Each component formed on the circuit board 15 is arranged in a cavity portion of a ceramic package 110, for example. A plurality of internal pads 120 are provided in the cavity portion of the package 110. The internal pad 120 is connected to a pad portion 75 of a heater wiring formed on the glass substrate 71 of the alkali metal cell 70 by a wire 131.

  The internal pad 120 is connected to the wiring of the photodetector 80 by a wire 132. Similarly, the wiring of the laser light emitting element 30 and the wiring of the heater for the laser light emitting element 30 are connected to the internal pad 120 by wire bonds or the like, respectively. Each internal pad 120 is electrically connected to the external terminal 129 through the wiring 125.

  After all the wirings are connected to the internal pads 120 of the package 110 as described above, for example, the ceramic lid 140 is disposed so as to be in contact with the outer peripheral portion of the package 110 and bonded in a high vacuum environment. Thereby, the inside of the package 110 and the lid 140 can be sealed in a high vacuum. For example, the surface of the region where the package 110 and the lid 140 are contacted is previously metallized, and a metal adhesion layer such as solder or AuSn is formed in each metallized region, and heated to a high temperature under high vacuum. Thus, the package 110 and the lid 140 can be bonded.

  The external terminal 129 is electrically connected to the modulator 200. The signal detected by the photodetector 80 is fed back to the modulator 200, and the modulator 200 can adjust the modulation frequency of the laser light from the laser light emitting element 30.

  By applying the CPT resonance detection method (pulse excitation method) according to the first embodiment to the atomic oscillator 10 according to the present embodiment, it becomes possible to reduce the light shift and improve the long-term stability. be able to. Moreover, since power broadening is suppressed and the resonance Q value is improved, high short-term stability can be obtained. Further, since a special light modulation device other than the laser light emitting element 30 is not required, the entire device can be reduced in size, cost, and power consumption can be reduced as compared with the case of using an external device.

<Third Embodiment>
In the third embodiment, a magnetic sensor including the CPT resonance generator according to the first embodiment as a basic configuration will be exemplified. Note that in the third embodiment, description of the same components as those of the already described embodiments may be omitted.

  A magnetic sensor can be realized by a configuration similar to that of the second embodiment (see FIG. 12). As shown in FIG. 8, the laser light passing through the excited Cs gas becomes maximum when the sideband frequency difference matches the natural frequency difference of the Cs atoms, but the natural frequency difference acts on the alkali metal cell 70. Shift by external magnetic field. Therefore, the value of the external magnetic field can be measured by measuring the resonance frequency.

  The amount of shift due to the external magnetic field differs depending on the magnetic quantum number mf of the hyperfine structure level that is Zeeman split. When the shift of the natural frequency difference by the external magnetic field is used as an atomic oscillator, it is desirable to use the clock transition of the magnetic quantum number mf = [0, 0] with the smallest shift amount of the natural frequency difference with respect to the magnetic field.

  On the other hand, when the shift of the natural frequency difference due to the external magnetic field is used as a magnetic sensor, a transition having a larger shift amount of the natural frequency difference with respect to the magnetic field (for example, the magnetic quantum number mf = [3, 3]) is used. It is desirable. In addition, the external magnetic field can be measured with high accuracy by measuring the frequency difference between the magnetic quantum numbers having different shift amounts.

  By applying the CPT resonance detection method (pulse excitation method) according to the first embodiment to the magnetic sensor according to the present embodiment, power broadening is suppressed and the resonance Q value is improved. A frequency difference can be detected with high accuracy. That is, when used as a magnetic sensor, the reading accuracy of the external magnetic field can be improved. Further, since no special light modulation device other than the laser light emitting element 30 is required, the entire device can be reduced in size, cost, and power consumption can be reduced as compared with the case where an external device is used.

  The preferred embodiment has been described in detail above. However, the present invention is not limited to the above-described embodiment, and various modifications and replacements are made to the above-described embodiment without departing from the scope described in the claims. Can be added.

DESCRIPTION OF SYMBOLS 1 CPT resonance generator 3, 30 Laser light emitting element 7, 70, 317 Alkali metal cell 9 Power supply device 10 Atomic oscillator 40 ND filter 50 Collimator lens 60 1/4 wavelength plate 80 Photo detector 200 Modulator 311 VCSEL
312 Current drive unit 313 Function generator 314 PLL
315 Bias circuit 316 Control unit 318 Photodiode 319 Lock-in amplifier 321 Sample hold circuit 322 Measuring instrument 323 Pulse generator

US Patent Application Publication No. 2013/0056458

Claims (13)

A method for generating CPT resonance in which laser light having at least two wavelengths is generated by current injection into a laser light emitting element, and an alkali metal is irradiated with the laser light,
The value of the direct current component of the current applied to the laser light emitting element is:
In the first period, greater than the oscillation threshold of the laser light emitting element,
In the second period following the first period, the value is smaller than the value of the direct current component of the current in the first period,
A method of generating CPT resonance, wherein Ramsey resonance is generated by repeating the first period and the second period a plurality of times.   2. The CPT resonance generation method according to claim 1, wherein a value of a direct current component of the current in the second period is smaller than an oscillation threshold value of the laser light emitting element.   3. The CPT resonance generation method according to claim 1, wherein the value of the direct current component of the current in the second period is 0. 4. In the first period,
The wavelength of the laser light is increased from a value smaller than the absorption wavelength of the alkali metal to a value larger than the absorption wavelength, and is decreased from a value larger than the absorption wavelength toward the absorption wavelength. The CPT resonance generation method according to claim 1, wherein the CPT resonance generation method varies. In the first period,
5. The current value applied to the laser light emitting element has a first value immediately after rising and a second value smaller than the first value. 6. The method for generating CPT resonance according to claim 1. A method for detecting CPT resonance generated by the CPT resonance generation method according to claim 4, comprising:
A CPT resonance detection method, wherein light that has passed through the alkali metal is detected when the wavelength matches the absorption wavelength in the increasing process.   The CPT resonance detection method according to claim 6, wherein the laser light emitting element is a surface emitting laser.   The CPT resonance detection method according to claim 6 or 7, wherein the alkali metal atom is sealed in a cell.   9. The CPT resonance detection method according to claim 6, wherein the alkali metal is any one of rubidium, cesium, sodium, and potassium. An alkali metal cell in which an alkali metal is enclosed and laser light having at least two wavelengths is incident;
A laser light emitting element for emitting the laser light;
A power supply device for applying a current to the laser light emitting element,
The value of the direct current component of the current applied to the laser light emitting element is:
In the first period, greater than the oscillation threshold of the laser light emitting element,
In the second period following the first period, the value is smaller than the value of the direct current component of the current in the first period,
The CPT resonance generator, wherein the first period and the second period are repeated a plurality of times.   The CPT resonance generator according to claim 10, wherein the laser light emitting element is a surface emitting laser.   An atomic oscillator comprising: the CPT resonance generator according to claim 10 or 11; and a light receiving unit that receives light that has passed through the alkali metal cell.   A magnetic sensor comprising: the CPT resonance generator according to claim 10 or 11; and a light receiving unit that receives light that has passed through the alkali metal cell.

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